The present invention relates to hardening materials or repairing living hard tissue which are applied to medical cements used for adhesion or mixing for repairing bone defects or gaps, dental pulp capping cements, dental root canal sealing cements, etc.
For medical cements used for adhesion and fixing for repairing bone defects and gaps in the orthopaedic surgery field, cyanoacrylate type adhesives and methyl methacrylate polymer-methyl methacrylate monomer type bone cements have been currently employed. These polymers however, are not able to integrate into hard tissue even after being retained in living body for long periods of time because their low biocompatibility. On the other hand, for root canal restoration in odontotherapy, the typical methods are of filling calcium hydroxide type pastes into root canal or of sealing root apex with zinc oxide eugenol cements together with gutta-percha points. However, there is a problem that the calcium hydroxide type pastes cause irritation because the pH of the neighborhood of the filled volume becomes too basic. Also, it has been pointed out in various fields that the zinc oxide eugenol cements show high cytotoxicity.
Recently, the granules and the block and porous forms of a calcium phosphate, hydroxyapatite [Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2 ; it may be called apatite hydroxide or HAP], have been clinically used as materials for repairing bone defects and gaps because of its high biocompatibility, on which many reports have been published.
These materials of hydroxyapatite, however, easily leak out from the incised part when filled into bone defects or gaps, because, although hydroxyapatite nontoxic to living body, it cannot integrate into hard tissue even after being retained in living body for long periods of time. In addition, since hydroxyapatite prepared either by dry method or by wet method does not cause a hardening reaction, it cannot be used as a substitute for the powder component of the above described cements.
Cement type materials, that is hardening materials, have exceedingly large possibility in applications of the materials because of their hardenability, adhesivity, moldability adaptable to complicated shapes and so on, unlike general ceramics (sintered substances).
Because of these reasons, hardening materials using biologically active calcium phosphates have been investigated. Thus, coagulation-hardening type materials, in which the phosphates harden by hydration reaction, and chelation-hardening type materials, in which the phosphates form chelate bonds with organic acids or carboxyl group-containing polymers contained in large amounts in the liquid component, resulting in hardening, have been proposed.
The coagulation-hardening type materials do not require of organic acids for hardening, because tile materials harden by the phenomena that the calcium phosphates cause hydration in water to coagulate and finally convert into hydroxyapatite or octacalcium phosphate [Ca.sub.8 H.sub.2 (PO.sub.4).sub.6.5H.sub.2 O; it may be called OCP]. Because of this reason, the coagulation-hardening type materials do not cause a lowering of pH due to the organic acids.
The biologically active calcium phosphates gradually convert into hydroxyapatite, which is a major component of living hard tissue, for example, in living body or oral cavity and Call integrate into living hard tissue. Alpha-tricalcium phosphate [Ca.sub.3 (PO.sub.4).sub.2 (alpha-type); it may be called alpha-TCP], tetracalcium phosphate [Ca.sub.4 (PO.sub.4).sub.2 O; it may be called TeCP or 4CP], and so on are exemplified as the phosphates. Various kinds of water-hardening calcium phosphate compositions using these calcium phosphates have been proposed.
For the coagulation-hardening type material using tetracalcium phosphate, a coagulation-hardening type dental restorative paste comprising a mixture of tetracalcium phosphate and calcium hydrogen phosphate is proposed. (U.S. Pat. No. 4,518,430).
The conventional coagulation-hardening type materials have problems that they show poor kneading-processability and cannot be filled uniformly because their pastes (paste may be called cement mud or kneaded product) do not spread uniformly. In general, hardening materials are used by suitably changing the ratio of their powder to liquid components in order to control setting time. If the amount of the powder component were increased in order to shorten the setting time, the kneading-processability and filling properties of the coagulation-hardening type materials would become further poor.
Because of the reason, the hardening materials which show desired effect only when the powder and liquid components are kneaded in a range of extremely limited ratio are not useful practically.
It was found by the inventors' research that since the pH of the neighborhood of the filled pastes of the conventional coagulation-hardening type materials are acidic or highly basic regions during hardening, the materials irritated the living body.